This invention relates to transducers, and particularly to electroacoustic, acoustoelectric, electromechanic, and mechanoelectric piezopolymer transducers, such as microphones, earphones, and loudspeakers for use in telephony and electrical communications, in broadcast, television and home recording applications, and other fields.
Piezopolymer transducers generally utilize a polymer-membrane sheet such as polyvinylidenefluoride composed of chain molecules with repeat units of CF.sub.2 CH.sub.2 referred to as PVDF or PVF.sub.2, polyvinylfluoride, polyvinylchloride, etc., as the piezoelectric material. Each face of the membrane or sheet is metallized for the application of potentials thereacross. In common with other piezoelectric devices, an electric field created by potentials across the electrodes formed by the metallization on the membrane surfaces produce distortions or other changes in the shape of the membrane material. Conversely, changing the shape of the membrane material produces an electric field detectable by connection to the electrodes.
Piezopolymer transducers constructed on the basis of the Bender principle are composed of a metallized polymer film of the type mentioned which is then curved and clamped at the edge. The purpose of the curvature is to achieve a desired linearity of transduction and proper matching between the transducer and the surrounding medium, e.g., air. Two methods of achieving the necessary curvature of the membrane have been suggested. One of these involves stretching the piezopolymer membrane over a spherical or other convex piece of foam rubber. This is described by M. Tamura, et al., in "Electroacoustic Transducers With Piezoelectric Films", in the journal of the Audio Engineering Society, Volume 23, page 21, (1975). According to another suggestion, the piezopolymer membranes were self-supported and achieved the advantage of higher sensitivity, particularly at lower temperatures. This is disclosed by R. Lerch in the article "Electroacoustic Transducers Using Piezoelectric Polyvinylidenefluoride Films" in J. Acoust. Soc. Am 66, 952 (1979). However, such transducers have a number of disadvantages.
The expected production tolerances and variations with foam rubber backings or with self-supported membranes are sufficiently high to significantly affect the membrane geometry. On the other hand, the membrane geometry substantially influences the sensitivity of such transducers. As a result, it is difficult to reproduce sensitivities, i.e., conversion factors, among transducers. Also, long term exposure to heat and other conditions may create undesired geometric deformations of the membrane and result in change of conversion factor over the age of a single transducer.
Therefore, piezopolymer transducers with foam rubber backing or self-supported transducers of this type exhibit lack of uniformity in production from the inherently large production tolerances or suffer from relatively poor long term stability. In addition, disadvantageous frequency shifts of resonances occur with devices of this type.
An object of the present invention is to eliminate the aforementioned drawbacks of such piezopolymer transducers.
Another object of the invention is to achieve better than hitherto available transduction from mechanical to electrical signals and vice versa with piezopolymer transducers.